1. Field of the Invention
The present invention relates to an image sensor used for a facsimile, scanner, and the like. More particularly, the present invention relates to a structure and a drive method for reducing a variation in the output signals among blocks each consisting of a plurality of photodetecting elements in an image sensor of the type in which signals of photodetecting elements are detected every block by a matrix drive system using switching elements connected to the photodetecting elements.
2. Discussion of the Related Art
Conventionally, a facsimile, for example, employs a close-contact type image sensor for converting the image information on a document original that is projected in one-to-one correspondence manner, into electrical signals. There has been proposed an image sensor of a TFT drive type. In the image sensor, the projected image is divided into a great number of picture elements (pixels), which respectively correspond to photodetecting elements. The charges generated by the respective photodetecting elements are temporarily stored into wire capacitors in every block by using switching elements such as thin film transistors (TFTs). The stored charges are time-sequentially read out in the form of electrical signals at a speed in the range of several hundred kHz to several MHz, by a drive IC. In this type of image sensor, the image read operation can be performed by using a single drive IC, through a matrix operation using the switching elements. Therefore, the number of drive ICs for driving the image sensor may be reduced.
The TFT drive image sensor, as shown in FIG. 5 showing an equivalent circuit of the image sensor, includes a linear photodetecting-element array 101 consisting of a plurality of photodetecting elements P.sub.k,n linearly arrayed and having a length substantially equal to the width of a document, a charge transfer section 102 consisting of a plurality of thin film transistors T.sub.k,n provided in association with the photodetecting elements P.sub.k,n in one-to-one correspondence, and matrix-arrayed signal transfer wires 103 constructed with a thin film structure.
The photodetecting element array 101 is divided into photodetecting element groups as "k" number of blocks. The "n" number of photodetecting elements P.sub.k,n forming one group may be each equivalently expressed by a photo diode PD and a parasitic capacitance Cp. The photodetecting elements P.sub.k,n are respectively connected to the drain electrodes of the switching elements T.sub.k,n. The source electrodes of the switching elements T.sub.k,n are respectively connected, every group of photodetecting elements, to the "n" number of common signal lines 104 through the signal transfer wires 103. The common signal lines 104 are connected to a drive IC 105. The gate electrodes of the switching elements T.sub.k,n are connected to a TFT controller 106 through control wires Gk so that the elements are rendered conductive every block. The control wires Gk are formed on an interlayer insulating film (not shown) that is formed on the signal transfer wires 103.
The optical charges generated in the photodetecting elements P.sub.k,n are stored, for a predetermined period of time, in the parasitic capacitance of each photodetecting element P.sub.k,n and the overlap capacitance CGD between the drain and gate of each switching element T.sub.k,n. Then, the charges are redistributed, every block, to the wiring capacitance CL of the signal transfer wires 103 and the overlap capacitance CGS between the source and gate of each switching element T.sub.k,n, by using the switching elements T.sub.k,n as charge transfer switches.
A gate pulse .PHI.G1 is transferred from the TFT controller 106, through the control wire G1 to the switching elements T.sub.1,1 to T.sub.1,n in the first block, so as to turn them on. The charges generated in the photodetecting elements P.sub.k,n of the first block are transferred to and stored in the wiring capacitances CL. By the charges stored in the wiring capacitances CL, the potential in the common signal lines 104 is varied. The varied voltages are time-sequentially introduced onto an output line 107 by successively turning on analog switches SWn in the drive IC 105. Further, in response to gate pulses .PHI.G2 to .PHI.Gk, the switching elements T.sub.2,1 -T.sub.2,n to T.sub.k,1 -T.sub.k,n in the second to k-th blocks are turned on, so that the charges of the photodetecting elements are transferred for every block. Thus, by simultaneously controlling the "n" number of switching elements every block, the signals of the "n" number of photodetecting elements are introduced in parallel to the drive IC 105. Then, by successively reading, every block, the potentials of the common signal lines 104 that are caused by the transferred charges produce an image signal for one line in the main scan direction on an original document. The original is moved by means of a document feed means (not shown), such as rollers, and the sequence of the operations as stated above is repeated. Finally, image signals of the whole document are obtained (Japanese Patent Application Unexamined Publication No. Sho. 63-9358). In addition, a switch RS is provided for removing the residual charge from the wiring capacitance CL, thereby to reset the capacitance CL.
In the operation of the image sensor for obtaining dark and halftone image signals, when the switching elements T.sub.k,n are turned on and off every block by the gate pulses .PHI.Gk from the TFT controller 106, the output signals of the photodetecting elements P.sub.k,n in several blocks after starting the gating operation tend to increase to be larger (in absolute value) than the output signals of the photodetecting elements in the subsequent blocks, as shown in FIG. 6. The tendency is marked when the operation is repeated under high temperature and high humidity conditions. This fact has been confirmed by a lot of experiments for reliability. In an experiment, the control wires G1 to G4 corresponding to the 1st to 4th blocks were disconnected, and the gating operation was started from the 5th block. An output signal increase tendency, which resembles that for the 1st to 4th blocks, was observed in the output signals of the photodetecting elements in the 5th block and the subsequent ones, as shown in FIG. 7.
The phenomenon of the signal output increase arises from the fact that in the initial several blocks after starting the gating operation, when gate pulses are applied to the control wires Gk, part of charges traveling through the signal transfer wires are stored in the interlayer insulating film (not shown) where the control wires Gk and the signal transfer wires 103 intersect, and apparently increased charges are transferred. Particularly, in the long-time operation under high temperature and high humidity conditions, the above phenomenon markedly appears because the dielectric constant of the interlayer insulating film is increased.
Thereafter, as the switching elements T.sub.k,n in the subsequent blocks become conductive in turn, the charge in the interlayer insulating film becomes saturated, while the amount of charge stored anew becomes reduced. The signal output read at the drive IC 105 approaches the amount of charge generated by the photodetecting elements P.sub.k,n.
In FIGS. 6 and 7, the characteristics were measured in the operation of reading dark output by the image sensor as stated above for about 100 hours and at a temperature of 85.degree. C. and a humidity of 85%. The curves show that the increase of the signal output is observed in the initial stage. The figures are exclusive of noise levels.
As seen from the foregoing description, in the TFT drive image sensor, the signal output apparently increases in the initial several blocks after the gating operation starts. The image signal read does not exactly represent the amount of charge generated by the photodetecting elements P.sub.k,n. Further, the increase of the signal output would reduce the prescribed lifetime of the sensor, leading to poor reliability of the sensor.